The efficiency of Na-K-ATPase in intact red blood cells, renal cells, several amphibian epithelial tissues and in vitro enzyme preparations in approximately 2 K+ pumped per ATP hydrolyzed. In contrast, studies on the reconstituted Na-K- ATPase from transformed cells have indicated that this enzyme is inefficient in these cells (i.e. approximately 0.2 K+ pumped/ATP hydrolyzed) and has been speculated to be the source of ADP and phosphate driving glycolysis at a high rate in transformed cells. We have studied the efficiency of Na-K-ATPase in intact transformed cells and found that the enzyme pumps approximately 2K+/ATP or has the same efficiency as in normal cells. In conclusion, we find no evidence, in the intact cell, that Na-K-ATPase is inefficient in transformed cells and that it is unlikely that the inefficient turnover of this pump is responsible for the enhanced lactate production of these cells.